Textline Detection

Detect textlines in document images

Introduction

This tool performs printspace, region and textline detection from document image data and returns the results as PAGE-XML. The goal of this project is to extract textlines of a document to feed an ocr model. This is achieved by four successive stages as follows:

• Printspace or border extraction
• Layout detection
• Textline detection
• Heuristic methods
  First three stages are done by using a pixel-wise segmentation. You can train your own model using this tool (https://github.com/qurator-spk/sbb_pixelwise_segmentation).

Printspace or border extraction

From ocr point of view and in order to avoid texts outside printspace region, you need to detect and extract printspace region. As mentioned briefly earlier this is done by a binary pixelwise-segmentation. We have trained our model by a dataset of 2000 documents where about 1200 of them was from dhsegment project (you can download the dataset from here https://github.com/dhlab-epfl/dhSegment/releases/download/v0.2/pages.zip) and the rest was annotated by myself using our dataset in SBB.
This is worthy to mention that for page (printspace or border) extractation you have to feed model whole image at once and not in patches.

Layout detection

At this point we look for textregions, that is why that we needed to do a layout detection. Here again a pixel-wise segmentation is implemented. For this purpose we had to provide training images and labels. In SBB we have a good resources and gt for layout of documents. By historical documents we have some main regions like , textregion, separators, images, tables and background and each has its own subclasses. For example for textregions we have subclasses like header, heading, drop capitals , main text and etc. As we can see we have many classes and in fact the ideal is to classify documents based on all those classes. But this is really a tough job and since here our focus is on ocr we decided to train our model with main regions including background, textregions, images and separators.
We have used 131 documents to train our model. Of course augmentation also hase done but here I do not want to explain training process in detail.

Textline detection

Last step is to do a binary pixelwise segmentation in order to classify textline pixels in document. For textline segmentation we had GT of documents with only one columns. This means that scale of documents were almost same , we tried to resolve this by feeding model with different scales of documents. However, even with this augmentation it was not easy to cover all spectrum of scales. So, this time we tried to use trained model and with tuning the parameters for multicolumns documents detect textlines. We then used this results also as GT to train new model which was much more robust.

Heuristic methods

After training models, we have used them to predict and classify documents in each step and then tried to use results to extract desirable textlines recatngles.
After applying page extraction model we then found the biggest contour and after fitting a bounding box we were able to crop image inside this box.
By layout detection, it was so important for us to detect textregions clearly separately that is why we have scaled image up. With this trick it was easier for model to detect background spaces between textregions.
We have set a minimum textregion area in respect to area of whole image, so those small textregions which are actullay noises in our prediction are filtered. At the end we have found contours of textregions and corresponding boundin boxes.
Textline segmentation is also done and using bounding boxes from textregions we are now able to get textline segmentation for each individual textregion. The first thing that we face by historical documents is that documents are skewed and even worser that each textregion can be skewed in a differnt manner. So, it was a key feature to deskew each textregion. Actually we have used textline segmengtation in each region to deskew corresponding region. After deskewing , calculating distribution of textlines segmentation result in X-direction helped us to find starting and ending point of every single textline. You can imagine that the hills in mentioned distribution are actully where we have background and no textline. Finally, using this coordinates we were able to find bounding rectangle for each textline.

Installation

pip install .

Models

In order to run this tool you also need trained models. You can download our pretrained models from here:
https://qurator-data.de/sbb_textline_detector/

Usage

sbb_textline_detector -i <image file name> -o <directory to write output xml> -m <directory of models>

Usage with OCR-D

ocrd-example-binarize -I OCR-D-IMG -O OCR-D-IMG-BIN
ocrd-sbb-textline-detector -I OCR-D-IMG-BIN -O OCR-D-SEG-LINE-SBB \
        -p '{ "model": "/path/to/the/models/textline_detection" }'

Segmentation works on raw RGB images, but retains AlternativeImages from binarization steps, so it's OK to do binarization first, then perform the textline detection. The used binarization processor must produce an AlternativeImage for the binarized image, not replace the original raw RGB image.